Grinding machine with feed rate changing apparatus

ABSTRACT

A grinding machine includes means for changing the feed rate of a grinding wheel relative to a workpiece when the grinding wheel approaches the workpiece. The grinding wheel is insulatedly mounted on a wheel slide which is moved by a feed device toward and away from the workpiece. An electrode device is mounted adjacent to the grinding wheel and is connected to an electric power source for generating a difference of potential between the grinding wheel and the workpiece. A detecting device detects the change of the difference of potential caused by the approach of the grinding wheel to the workpiece to generate a control signal. A control device controls the feed device so as to change the feed rate of the wheel slide in accordance with the control signal.

Tomita et a1.

[451 Sept. 16, 1975 GRINDING MACHINE WITH FEED RATE CHANGING APPARATUS Inventors: Tamaki Tomita, Okazaki; Hayashi Kodama, Kariya, both of Japan [73] Assignee: Toyoda Koki Kabushiki Kaisha, Japan [22] Filed: May 20, 1974 [21] Appl. No.: 471,559

[30] Foreign Application Priority Data 7 [56] References Cited UNITED STATES PATENTS 2,961,808 11/1960 Dunigan 5l/165.92 3,090,171 5/1963 Stimson... 51/165 R 3,172,240 3/1965 Giardini 5l/l65.92 X 3,550,327 12/1970 Kusakabe 51/l65.92

R 2.5 I 4:: DIRECT CUM/IT SMIRCE FOREIGN PATENTS OR APPLICATIONS 1,371,307 7/1963 France 5l/l65.77 782,432 9/1957 United Kingdom 5l/16S.92

Primary ExaminerHarold D. Whitehead Attorney, Agent, or Firm-Oblon, Fisher, Spivak, McClelland & Maier [5 7 ABSTRACT A grinding machine includes means for changing the feed rate of a grinding wheel relative to a workpiece when the grinding wheel approaches the workpiece.

The grinding wheel is insulatedly mounted on a wheel slide which is moved by a feed device toward and away from the workpiece. An electrode device is mounted adjacent to the grinding wheel and is connected to an electric power source for generating a difference of potential between the grinding wheel and the workpiece. A detecting device detects the change of the difference of potential caused by the approach of the grinding wheel to the workpiece to generate a control signal. A control device controls the feed device so as to change the feed rate of the wheel slide in accordance with the control signal.

15 Claims, 7 Drawing Figures wnllllllllllllllllll ll PATENTEU SEP I 61975 SHEET 1 [IF 5 BR NAR GRINDING MACHINE WITH FEED RATE CHANGING APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention v This invention relates generally to grinding apparatus, and more particularly to a grinding machine provided with an apparatus for changing the feed rate of a grinding wheel relative to a workpiece when the grinding wheel approaches or contacts the workpiece.

2. Description of the Prior Art For the purpose of detecting the contact of a grinding wheel with a workpiece, conventional feed ratechanging devices utilize a change of input current of a grinding wheel-driving motor caused by the contact of the grinding wheel with the workpiece, a change of moment of the rotating grinding wheel, or a vibration or a noise caused by the contact of the grinding wheel with the workpiece. The feed rate changing devices using such detecting methods can not, however, change the feed rate of the grinding wheel just before or at the moment.

Another method that detects the contact of the grinding wheel with the workpiece utilizes a conductive circuit formed by the contact between a grinding machine and a conductive grinding wheel. However, this method suffers from the drawback that a nonconductive grinding wheel can not be utilized.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a grinding machine provided with new and improved apparatus for changing the feed rate of a grinding wheel just before the grinding wheel contacts the workpiece. I

It is another object of the present invention to provide a grinding machine provided with new and improved apparatus wherein a difference of potential is generated between a grinding wheel and a workpiece and the change of the difference of potential caused by the approach of the grinding wheel to the workpiece is detected to chm ge the feed rate of the grinding wheel.

Other objects will appear hereinafter.

According to the present invention these and other objects are achieved by a grinding machine which comprises a bed, a wheel slide slidaby mounted on the bed, a wheel shaft rotatably mounted on the wheel slide, a grinding wheel attached to the wheel shaft, a drive motor for rotating the grinding wheel, a work table slidably mounted on the bed for supporting a workpiece and a feed device for moving the wheel slide toward and away from the workpiece. An electrode device is mounted adjacent to the grinding wheel, which is in non conductive relationship with the wheel shaft, and is connected to an electric power source for generating a difference of potential between the grinding wheel and the workpiece. A detecting device detects the I change of the difference of potential caused in-the approach of the grinding wheel to the workpiece to generate a control signal. A control device controls the feed device so as to change the feed rate of the wheel slide in accordance with the control signal.

BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects of the present invention will be more fully appreciated as the same becomes better understood from the following detailed description of preferred embodiments when considered in connection with the accompanying drawings, in which:

FIG. I is a schematic view showing a first embodiment according to the present invention;

FIG. 2 is a relay circuit;

FIG. 3 is a schematic view showing a second embodi ment; i

' FIG. 4 is a cross sectional view, taken along the line IVIV of FIG. 3;

FIG. 5 is a cross sectional view showing another embodiment of a nozzle;

FIG. 6 is a schematic view showing a third embodiment; and

FIG. 7 is a schematic view showing a fourth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, wherein like reference numerals and signs refer to identical or corresponding parts throughout the several views, a grinding machine in accordance with the present invention is described with reference to FIGS. 1, 3, 6 and 7 which show four different embodiments of the present invention.

A work table 2 is slidably mounted on a bed 1 which fixedly mounts a head stock 3 and a tail stock (not shown) for rotatably supporting a workpiece W therebetween. A wheel slide 4 is slidably mounted on the bed 1 so as to be moved toward and away from the supported workpiece W. The above-mentioned grinding machine parts are made of conductive material. A grinding wheel G is fixed to a wheel shaft 5, made of conductive material, and is in non-conductive relationship with the wheel shaft 5. The wheel shaft'S is rotatably mounted on the wheel slide 4 and is drivingly connected to a drive motor 7 through a belt 6. In case a nonconductive grinding wheel, such as, for example a vitrified grinding wheel is mounted on the wheel shaft 5,1it is not necessary to interpose an insulated material between the grinding wheel G and the wheel shaft 5. Accordingly, in both cases, an electric condenser is created between the grinding wheel G and the workpiece W.

A feed cylinder 11 is mounted on the bed 1 for slidably receiving a piston. A piston rod 13 is connected to the lower portion of the wheel slide 4. Fluid under pressure is applied to and is discharged from the cylinder 11 through a magnetic change-over valve 14 for changing the advance and the retraction of the wheel slide 4.

' Another change-over valve 15 is used for changing the feed rate of the wheel slide 4. A check valve 16 is connected to and between the cylinder 11 and the changeover valve 14 in parallel with the change-over valve 15 for causing the wheel slide 4 to retract rapidly.

A coolant nozzle 8 is secured on the top of a grinding wheel guard 12 for discharging a conductive coolant to the grinding wheel G and the workpiece W during a grinding operation. A pump 17 is connected to the coolant nozzle 8 by means of a conduit 9 for supplying the conductive coolant from a coolant reservior 18 to the coolant nozzle 8. A magnetic Chang-over valve 10 is connected to andbetween the coolant nozzle 8 and the pump 17 for opening and closing the conduit 9.

The first embodiment is described in detail below with reference to FIG. 1.

A conductive nozzle 20 is fixed to the grinding wheel guard 12 so as to be electrically insulated from the same for discharging the conductive coolant into the periphery of the grinding wheel G to thereby wet the same. The conductive nozzle 20 which serves also as an electrode to provide electric power is connected to the conduit 9 through a relatively long nonconductive pipe 21 for preventing leakage of current to the conductivity of coolant. Conduit 9 is connected to the pump 17. A variable throttle 22 is interposed between the conductive nozzle 20 and the conduit 9 for adjusting the quantity of the coolant applied to the grinding wheel G so as not to wet the vicinity of the wheel shaft and so as to apply an appropriate coolant layer to the periphery of the grinding wheel G.

A direct current source 25 is used as an electric power device in this embodiment. The direct current source 25 is connected at its positive output terminal to the conductive nozzle 20 through a variable resistance R and is grounded to the head stock 3 at its negative output terminal. An amplifier 27 is connected at its positive input terminal to the variable resistance R and the conductive nozzle 20 and is grounded at its nega tive input terminal for amplifying a difference of potential appearing between the heat stock 3 and the nozzle 20 to generate an output voltage.

A first setting device 30 sets a first reference voltage equal to the output voltage appearing at the output terminal of the amplifier 27 when a predetermined gap is formed between the outer peripheral edge of the grinding wheel G and the workpiece W during the rapid advance of the wheel slide 4. A first comparator circuit 28 is connected to the output terminal of the amplifier 27 and the first setting device 30 for comparing the output voltage of the amplifier 27 with the first reference voltage to thereby generate a control signal when the output voltage falls to the first reference voltage. The output terminal of the first comparator circuit 28 is connected to a relay circuit 40, described hereinafter, which changes the magnetic change-over valves and to their ports I in accordance with the control signal. As mentioned above, a feed rate-changing device is constituted by the first setting device 30, the first comparator circuit 28, the amplifier 27, the relay circuit 40 and the magnetic change-over valve 15.

A second setting device 31 sets a second reference voltage equal to the output voltage appearing at the output terminal of the amplifier 27 when the periphery of the grinding wheel G is in the desired wet state. A second comparator circuit 29 is connected to the output terminal of the amplifier 27 and the second setting device 31 for comparing the output voltage of the amplifier 27 with the second reference voltage to thereby generate a confirmation signal when the output voltage attains the second reference voltage. The output terminal of the second comparator circuit 29 is connected to the relay circuit 40 to change the magnetic changeover valve 14 to its port 11 in accordance with the confirmation signal.

A safety device, used also in the other embodiments, for preventing the grinding wheel G from colliding with the workpiece W at the rapid feed rate when the feed rate-changing device malfunctions is described hereunder.

A current transformer 35 is connected to the power input terminal of the drive motor 7 for transforming a high input alternating current applied to the power input terminal to a predetermined low current adequate for the components of the safety device. When an excessive load is applied to the grinding wheel G, an excessive current appears at the power input terminal of the drive motor 7 and is detected by the safety device. A rectifier 36 is connected to the current transformer 35 for converting the alternating current to a direct current. A setting device 38 sets a safety reference current for preventing a collision of the grinding wheel G with the workpiece W. A comparator circuit 37 is connected to the setting device 38 and the rectifier 36 for comparing the direct current transmitted from the rectifier 36 with the safety reference current to thereby generate a safety signal when the direct current becomes larger than the safety reference current. The comparator circuit 37 is connected at its output terminal to the relay circuit 40 to change the magnetic change-over valve 14 to its port I in accordance with the safety signal.

The relay circuit 40 is described hereunder with reference to FIG. 2 wherein a sign A designates a normally open contact closed in accordance with the confirmation signal, a sign B designates a normally open contact closed in accordance with the control signal, a sign C designates a normally open contact closed in accordancewith the safety signaland a sign D designates a normally open contact closed in accordance with a retraction command signal, described hereinafter. Normally open contacts Crl to Cr4 are adapted to be closed in accordance with energizations of relays CR1 to CR4, respectively. Normally closed contacts Cr2x and CrSx are adapted to be opened in accordance with energizations of relays CR2 and CR5, respectively. A sign PBl designates an emergency push button.

The operation of the abovementioned first embodiment is described hereunder.

FIG. 1 shows a state before a grinding operation in which the wheel slide 4 and the other accompanying parts are in their initial positions or states. The drive motor 7 is operated to rotate the grinding wheel G and the pump 17 is driven to supply the coolant to the nozz le 20 through the variable throttle 22. Since the periphery of the grinding wheel G is brought into the wet state, the direct current source 25 is electrically connected to the periphery of the grinding wheel through the nozzle 20 and the coolant. Because of the application of the coolant having a large resistivity and the relatively long nonconductive pipe 21, an electric conduit formed by the pipe 21 and the coolant is given a large electrical resistance, so that the coolant nozzle 20 is effectively insulated from the coolant reservior 18. Also, the grinding wheel G is electrically isolated from the wheel shaft 5 as described hereinbefore. Thus, since a leakage current from the direct current source 25 is effectively suppressed, a voltage closely approximating the output voltage of the direct current source 25 appears at the nozzle 20.

The voltage at the nozzle 20 is amplified by the amplifier 27 and is in turn applied to comparator circuits 28 and 29 to be compared with the first and the second reference voltages. When the grinding wheel G is in a sufficiently wet state and is not electrically shorted with the wheel shaft 5, the output voltage of the amplifier 27 is greater than the second reference voltage. Conversely, when the periphery of the grinding wheel G is electrically shorted with the wheel shaft 5, the difference of potential between the coolant nozzle 20 and the head stock 3 decreases and the output voltage of the amplifier 27 decreases below the second reference voltage with the result that the confirmation signal is not transmitted from the second comparator circuit 29.

When the start push buttom PB2 is operated after the confirmation signal is generated, the relay CR1 is energized to thereby close its self-holding contact Crl. When the contact B is closed by the confirmation signal, the relay CR3 is energized to thereby energize the solenoid of the magnetic change-over valve 14 and change the magnetic change-over valve 14 to its port ll. Then, fluid under pressure is applied to the right chamber of the feed cylinder 11 to thereby move the wheel slide 4 toward the workpiece at the rapid feed rate. The relay CR4 for the rapid feed rate is energized by the contact Cr3 which is closed by the energization of the relay CR3. When thecoolant film formed on the periphery of the grinding wheel G is brought into contact engagement with the workpiece W by the rapid advance of the wheel slide 4, the conductive nozzle 20 is electrically shorted with the workpiece W or the head stock 3 through the coolant film. Accordingly, the difference of potential applied to the amplifier 27 rapidly decreases and theoutput voltage of the amplifier 27 decreases below the first reference voltage causing a control signal to be transmitted from the first comparator circuit 28. The contact A is closed in accordance with the control signal to thereby energize the relay CR2 for a slow or grinding feed rate. The magnetic change-over valve is changed to its port I by the energized solenoid thereof to thereby supply considerably coolant for the grinding operation through the coolant nozzle 8. At the same time that the magnetic changeover valve is changed to its port I by the energized solenoid thereof, the wheel slide 4 is advanced at the slow or grinding feed rate and the relay CR4 for the rapid feed rate is deenergized by the opening of the normally closed contact Cr2x.

As mentioned above, the feed rate of the wheel slide 4 is changed to the slow feed rate just before the grinding wheel G is brought into direct contact with the workpiece W which is ground by the grinding wheel G at the changed slow feed rate. When the workpiece W is ground and finished to a predetermined dimension and shape, the retraction command signal is transmitted from an appropriate device (not shown), such as, e.g. a sizing device, to thereby close the contact D. The relay CR5 for commanding the retraction of the wheel slide 4 is energized to thereby deenergize relays CR1, CR2 through their normally closed contacts CrSx and in turn to deenergize the relay CR3 through the opening of the contact Crl. The above-described magnetic changeover valves, 10, 15 and 14, the solenoids of which are deenergized, are changed to their initial ports to thereby stop the supply of coolant and to thereby move the wheel slide 4 away from the workpiece W. Thus, all elements return to their initial positions as shown in FIG. 1.

Since the input current of the drive motor 7 increases because of the grinding resistance due to the cutting engagement of grinding wheel G with the workpiece W, the output current value of the rectifier 36 increases beyond the safety reference current so that the contact C is closed. Accordingly, when the aforementioned feed rate changing device malfunctions, for example, if the coolant is not supplied from the coolant nozzle or the rapid feed rate of the wheel slide 4 is not changed to the slow feed rate when the grinding wheel G is engaged with the workpiece W, the relay CR5 is energized to rapidly move the wheel slide 4 away from the workpiece W because the contact C is closed in addition to the still closed contact Cr4.

The experimental results of theabovementioned first embodiment are described below.

When the peripheral speed of the grinding wheel G is set to 30 meters/second and the output voltage of the direct current source 25 is changed between 30 and 35 volts,

Vo 10-12 volts Vc 7-9 volts wherein Vo designates the voltage appearing at the input terminal of the amplifier 27 in the disengagement state of the grinding wheel G with the workpiece W and Vc designates the voltage appearing at the input terminal of the amplifier 27 when 'the coolant film on the grinding wheel G contacts the workpiece W.

The aforementioned feed rate-changing device is not affected in its operation even when the coolant discharged from the conductive nozzle 20 electrically connects the lower portion of the grinding wheel G with the grinding wheel guard 12 or the bed I by its continuous fall because the falling coolant has a high resistance.

An alternating current source such as an oscillator may be applied in substitution of the direct current source 25. The conductive nozzle 20 and the coolant nozzle 8 may be combined into a single nozzle which is adapted to supply a different quantity of coolant during the operating time of the feed rate-changing device and during the grinding operation time. The coolant nozzle 8 may be fixed to the lower portion of the grinding wheel guard 12 so as todischarge coolant upwardly into the grinding point.

The second embodiment of the present invention is described hereunder with reference to FIG. 3 in which certain above-mentioned parts are omitted in the description of the second embodiment.

An electrode 250 is fixed to the grinding wheel guard 12 so as to be electrically insulated from the same through an insulated member 19 which opposes the side of the grinding wheel G with a small clearance as best shown in FIG. 4. The electrode 250 may be disposed opposite to the periphery of the grinding wheel G, although the electrode 250 must allow the decrease of the diameter of the grinding wheel G so that the clearance between the electrode 250 and theperiphery of the grinding wheel G is kept constant. Furthermore, electrodes 250 may be fixed on both sides of the grind- 'ing wheel G.

The electrode 250 is connected to one of the output terminals of an oscillator 251 through a variable resistance R, the other output terminal being grounded on the head stock 3 or the bed 1. The amplifier 27 is connected at one terminal to the electrode 250 and the variable resistance R for detecting and amplifying the difference of potential between the grinding wheel and the workpiece. The other terminal of amplifier 27 is grounded to the bed 1. The output voltage of amplifier 27 is applied to a rectifier 252 which converts an alternating current to a direct current. The output of rectifier 252 is connected to the inputs of the same comparator circuits 28 and 29 as is shown in the first embodiment. Descriptions for first and second setting devices 30 and 31 and the relay circuit 40 are omitted because the same devices and circuit are described in the first embodiment.

The nozzle 20 is connectd to an atomizer 253 to receive a conductive atomized coolant to adequately wet only the peripheral portion of the grinding wheel G. The atomizer 253 comprises a filter 254 for cleaning compressed air supplied from an air pump (not shown), a regulator 255 for maintaining the compressed air constant and a mist making device 256 for atomizing coolant supplied by the compressed air.

The operation of the second embodiment as described hereunder.

The compressed air is applied to the atomizer 253 to make the atomized coolant, which is discharged from the nozzle 20 into the periphery of the rotating grinding wheel G to thereby wet the peripheral portion. Then, since the vicinity of wheel shaft is not wetted by the operation of centrifugal force of the grinding wheel, an insulated state is maintained between the grinding wheel G and the wheel shaft 5. The peripheral portion of the grinding wheel G is rendered conductive by being wetted by the applied coolant. As a result, a condenser is formed by the grinding wheel G and the electrode 250 between which an alternating current can be conducted.

The grinding wheel G is, accordingly, grounded by the coolant film formed thereon in its approach to the workpiece, an electric conduit being formed by the oscillator 251, the variable resistance R, the electrode 250, the coolant film, the workpiece W and the bed 1. Then, the difference of potential applied to the amplifier 27 decreases so that coolant is supplied from the coolant nozzle 8. The feed rate of the wheel slide 4 is changed to the slow feed rate by the operation of the comparator circuit 28 and the relay circuit 40 as aforementioned with respect to the first embodiment. The second comparator circuit 29, also, performs the same operation as is described in the first embodiment.

Although the second embodiment is constituted in such a manner that the coolant nozzle delivers the atomized coolant to the grinding wheel G, an adequate amount of coolant, not atomized, may be delivered to the grinding wheel G as well. The nozzle 20 may be omitted if the peripheral portion of the grinding wheel G can be maintained adequately wet by the coolant supplied in the previous grinding operation for a short time from the accomplishment of a previous grinding operation to the start of the next grinding operation. In the experiment, it was found that the peripheral portion ofgrinding wheel G was still adequately wet for 2 minutes after the cessation of the coolant supply. The atomized coolant may be discharged toward the peripheral side surface of the grinding wheel or toward both the peripheral and the side thereof in accordance with the peripheral speed of the grinding wheel G as shown in FIG. 5.

For the purpose of maintaining the peripheral portion of the grinding wheel G wet so as to cause the second embodiment to operate normally, the atomized coolant may be discharged (1) to the outer peripheral edge or the side of the grinding wheel G at a peripheral speed of 30 meters/second, (2) to both sides of the grinding wheel G at a peripheral speed of 45 meters/- second and (3) to both sides and the outer-peripheral edge at a peripheral speed of 60 meters/second.

The above-mentioned modifications with respect to the second embodiment can be applied to the third embodiment described hereunder.

FIG. 6 shows the third embodiment, which is different from the second embodiment only in the detection of the difference of potential between the grinding wheel G and the workpiece W. Therefore, only this difference is described in detail hereunder.

A second electrode 360 is secured to the grinding wheel guard 12 so as to be electrically isolated from the same through an isolated member 19 which opposes the peripheral and side portion of grinding wheel G with a small clearance. Second electrode 360 is insulated from the first electrode 250, mentioned hereinbefore, which is connected only to the oscillator 251 through the variable resistance R. The second electrode 360 may be secured so as to be opposed to the outer periphery or opposed to both sides of the grinding wheel G as well as the first electrode 250. Furthermore, first and second electrodes250 and 360 may be positioned at upper and lower positions, respectively, so as to put the grinding point therebetween. Both electrodes 250 and 360 may be disposed on the same side, for example, on the upper or the lower side, with respect to the grinding point.

When the peripheral portion of grinding wheel G is wet and thereby conductive, a condenser is formed by the first electrode 250, the second electrode 360 and the wet peripheral portion of the grinding wheel G. Therefore, an alternating current which is transmitted from the oscillator 251 flows between the first electrode 250 and the second electrode 360 through the wet peripheral portion of the grinding wheel G. The difference of potential caused between the second electrode 360 and the bed 1 is applied to the amplifier 27 to be amplified.

When the coolant film encircling the grinding wheel G contacts with or approaches the workpiece W causing an electrical short circuit to the. bed 1, the difierence of potential between the second electrode 360 and the bed 1 is markedly decreased. In accordance with the change of the difference of potential, the coolant is supplied from the coolant nozzle 8 and the rapid feed rate of the wheel slide 4 is changed to the slow or grinding feed rate.

The experimental results of the above-mentioned third embodiment are described below.

When the peripheral speed of the grinding wheel G is at 30 meters/second and 45 meters/second and the output voltage of the oscillator 251 is changed between 30 and 35 volts,

V0 30-50 milli-volts Vc 0-10 milli-volts I wherein V0 designates the voltage appearing at the input terminal of amplifier 27 in the disengagement state of the grinding wheel G with the workpiece W and V0 designates the voltage appearing at the input tenninal of the amplifier 27 when the coolant film on the grinding wheel G approaches and contacts the workpiece W.

The fourth embodiment is described hereunder with reference to FIG. 7. This embodiment is constituted, as mentioneed below, so as to prevent the aforementioned confirmation signal from being transmitted because of the electrical connection of the grinding wheel G with the workpiece W which is effected by considerable conductive coolant scattered about the grinding wheel G, notwithstanding that the grinding wheel G is relatively distant from the workpiece.

The nozzle 20 is mounted on the lower portion of the grinding wheel guard 12 below the grinding wheel G so as to be electrically insulated from the grinding wheel guard 12. The conductive coolant discharged from the nozzle 20 is carried by the rotation of grinding wheel G to the grinding position through the back of i the grinding wheel G. Thus, the conductive coolant applied to the workpiece W at the grinding position is reduced to a suitable amount. Furthermore, a baffle plate 400 is attached to the lower portion of a front cover 12a which is mounted on the grinding wheel guard 12 for covering the upper front of the grinding'wheel G for preventing the coolant from scattering to the front of the grinding wheel G. The baffle plate 400 is effectively used when the peripheral speed of the grinding wheel G is below 30 meters/seconds but is not necessary when the peripheral speed of grinding wheel G is over 30 meters/second because almost all the coolant is displaced toward the back of grinding wheel G. The baffle plate 400 is operable to intercept an air film rotating with the grinding wheel G for ensuring a positive supply of coolant to the grinding point of the grinding wheel G.

The nozzle 20 is insulatedly fixed to the grinding wheel guard 12 and is connected to the reservoir 18 through the pump 17. One output terminal of oscillator 251 is connected to the nozzle 20 by variable resistance R, the other output terminal being grounded.

An electrode 450 is mounted on the grinding wheel guard 12 so as to be opposed. to the side of the grinding wheel G and separated therefrom by a small clearance. The electrode 450 provides a condenser having a grinding wheel G as an opposite electrode as well as the electrode 250 of the second embodiment, shown in FIG. 3. The amplifier 27 is connected to and between the electrode 450 and the bed 1 for amplifying the difference of the potential detected therebetween. The amplifier 27 is connected to the rectifier 252 which is connected to first and second comparator circuits 28 and 29 which are connected to first and second setting devices 30 and 31, respectively. First and secondcomparator circuits=28 and 29 are connected to relay circuit 40.

When the peripheral portion of the grinding wheel G is wet from the coolant discharged from the nozzle 20, the alternating output voltage of the oscillator 251 appears at the electrode 450 through the condenser which is constituted by nozzle 20, the coolant, the grinding wheel G and the electrode 450. This voltage is applied to the amplifier 27. The output voltage of the electrode 450 is markedly reduced when the coolant film encircling the grinding wheel G contacts or approaches the workpiece W. Then, the rapid feed rate of wheel slide 4 is changed to the slow or grinding feed rate. I

The experimental results of the above-mentioned fourth embodiment are described below.

When the peripheral speed of the grinding wheel G is set to 30 meters/second, 45 meters/second and 60 meters/second and the output voltage of the oscillator 251 is changed between 30 and 35 volts,

Vo 200-400 milli-volts Vc 801 50 mille-volts wherein V designates the voltage appearing at the input terminal of the amplifier 27 during the disengagement state of grinding wheel G with workpiece W and Vc designates the voltage appearing at the input termina] of the amplifier 27 when the coolant film encircling the grinding wheel G contacts andapproaches workpiece .W.

ln the aformentioned embodiments, the insulated member may be provided for covering the wheel shaft 5 to prevent the insulation between the wheel shaft 5 or the wheel slide 4 and the grinding wheel G from being broken by coolant wetting the wheel shaft 5. Furthermore, in the third and fourth embodiments, the oscillator 251 and the amplifier 27 may be reversely connected to the two elements. In the fourth embodiment, another nozzle as well as nozzle 20 may be disposed in substitution of the electrode 450. 7

'Although certain specific embodiments of the, invention have been shown and described, it is obvious that many modifications and variations thereof are possible in light of the above teachings. It is understood, therefore, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

What is claimed as new and desired to be secured by Letters Patent of the United States is: I

' l. A grinding machine comprising:

a bed; a wheel slide slidably mounted on said bed; a wheel shaft rotatably mounted on said wheel slide; a grinding wheel attached to said wheel shaft with electrically non-conductive relationship therebetween; a drive motor for rotating said grinding wheel; a work table slidably mounted on said bed for supporting a workpiece; feed means for moving said wheel slide toward and away from said workpiece; an electric power source; electrode means mounted adjacent to said grinding I wheel and connected to said electric power source for generatinga difference of potential by creating an electric condenser between said grinding wheel and said workpiece;

coolant means for connecting said electrode means and said grinding wheel so as to charge said grinding wheel by creating a coolant layer thereon;

detecting means for detecting the change of the difference of potential caused by engagement between said coolant layer on said grinding wheel and said workpiece to generate a control signal; and

-. control means for controlling said feed means'so as to change the feed rate of said wheel slide in accordance said control signal. 2. A grinding machine according to claim 1, further comprising a safety means connected to said control means and said drive motor for preventing said grinding wheel from being damaged by colliding with said workpiece when said electrode means or said detecting means malfunction. I i

.3. A grinding machine according to claim 2, wherein said safety means comprises:

a current transformer connected to said drive motor for transforming a high input current applied to said drive motor to a low alternating current;

a rectifier for rectifying said low alternating current to generate a direct current, said direct current increasing when an excessive load is applied to said grinding wheel;

a setting means for setting a safety reference current;

and

a comparator circuit connected to said rectifier. and

said setting means for comparing'said direct current with said safety reference current to generate a safety signal when said direct current attains said safety reference current, said control means being operable to cause said feed means to retract said wheel slide in response to said safety Signal.

4; A grinding machine according to claim 1, wherein said electrode means has a coolant nozzle of said coolant means for wetting said grinding wheelwith-a conductive coolant.

' 5. A grinding machine according to claim 4, wherein said detecting means comprises: a

an amplifier for amplifying the difference of potential to generate an output voltage;

afirst setting means for setting a firstreference voltage equal to the output voltage appearing at an output terminal of said amplifier when a predetermined gap is formed between an outer peripheral edge of said grinding wheel and said workpiece when said feed means moves said wheel slide toward said'workpiece at a rapid feed rate; and

a first comparator circuit connected to said amplifier and said first setting means for comparing said output voltage with said first reference voltage to generate said control signal when theoutput .of said amplifier voltage attains said first reference voltage, said control means causing said feed means to change the feed rate of said wheel slide from a rapid feed rate to a slow feed rate in accordance with said control signal. v

6. A grinding machine according to claim 5, wherein .said detecting means further comprises:

a second setting means for setting a second reference voltage equal to the output voltage appearing at an output terminal of said amplifier when said grinding wheel is in a desired wet state; and

a second comparator.circuitconnected to the output terminal of said amplifier and said second setting device for comparing the output voltage of said amplifier with said second reference .voltage to generate a confirmation signal, said control means being permitted in accordance with said confirmation signal to cause said feed means to move said wheel slide toward said workpiece.

7. A grinding machine according to claim 5, wherein said electric power-source is a direct current source.

8. A grinding machine according to claim 5, wherein said electric power source is an oscillator and further comprises a rectifier connected to and'between said amplifier and said first comparator.

9. A grinding machine according to'claim 8', wherein said electrode means comprises:

a pair of electrodes mounted adjacent to a periphery of said grinding wheel, one of said electrodes being connected to said oscillator and the other of said electrodes being connected to said amplifier;

an atomizer means for supplying an atomized coolant, and

a coolant nozzle mounted adjacent to the pereiphery of said grinding wheel for wetting thesame with said atomized coolant.

10. A grinding wheel according to claim 8, wherein said detecting means further comprises:

a second setting means for setting a second reference voltage equal to the output voltage appearing at an output terminal of said amplifier when said grinding wheel is in a desired wet state; and

a second comparator circuit connected to the output terminal of said amplifier and said second setting device for comparing the output voltage of said amplifier with said second reference voltage to generate a confirmation signal, said control means I being permitted in accordance with said confirmation signal to cause said feed means to move said wheel slide toward said workpiece.

11. A grinding machine according to claim 9,

wherein said electrode means comprises:

an electrode mounted adjacent to a periphery of said grinding wheel, said electrode being connected to said amplifier; and

a coolant nozzle mounted below said grinding wheel for supplying a conductive coolant, said coolant nozzle being connected to said oscillator.

12. A grinding machine according to claim 11,

wherein said detecting means further comprises:

a second setting means for setting a second reference voltage equal toth'e output voltage appearing at an output terminal of said amplifier when said grinding wheel is in a desired wet state; and a second comparator circuit to the output terminal of said amplifier and said second setting device for comparing the output voltage of said amplifier with said secondreference voltage to generate a confirmation signal, said control means being permitted in accordance with said confirmation signal to cause said feed means to move said wheel slide toward said workpiece. I

' 13. A grinding machine according to claim 1, wherein said coolant means is operable to supply coolant to said grinding wheel in response to said control signal.

X 14. A grinding machine according to claim 1, wherein said coolant means comprises:

a first means for creating a desired coolant layer on said grinding wheel so as to connect said electrode 1 means and said workpiece; and

a second means for supplying a large amount of coolant to said grinding wheel and the workpiece in response to said control signal.

15. A grinding machine according to claim 14, which further comprises a variable throttle valve-for controlling an amount of coolant supplied to said first means.

.- l= =l= l 

1. A grinding machine comprising: a bed; a wheel slide slidably mounted on said bed; a wheel shaft rotatably mounted on said wheel slide; a grinding wheel attached to said wheel shaft with electrically non-conductive relationship therebetween; a drive motor for rotating said grinding wheel; a work table slidably mounted on said bed for supporting a workpiece; feed means for moving said wheel slide toward and away from said workpiece; an electric power source; electrode means mounted adjacent to said grinding wheel and connected to said electric power source for generating a difference of potential by creating an electric condenser between said grinding wheel and said workpiece; coolant means for connecting said electrode means and said grinding wheel so as to charge said grinding wheel by creating a coolant layer thereon; detecting means for detecting the change of the difference of potential caused by engagement between said coolant layer on said grinding wheel and said workpiece to generate a control signal; and control means for controlling said feed means so as to change the feed rate of said wheel slide in accordance with said control signal.
 2. A grinding machine according to claim 1, further comprising a safety means connected to said control means and said drive motor for preventing said grinding wheel from being damaged by colliding with said workpiece when said electrode means or said detecting means malfunction.
 3. A grinding machine according to claim 2, wherein said safety means comprises: a current transformer connected to said drive motor for transforming a high input current applied to said drive motor to a low alternating current; a rectifier for rectifying said low alternating current to generate a direct current, said direct current increasing when an excessive load is applied to said grinding wheel; a setting means for setting a safety reference current; and a comparator circuit connected to said rectifier and said setting means for comparing said direct current with said safety reference current to generate a safety signal when said direct current attains said safety reference current, said control means being operable to cause said feed means to retract said wheel slide in response to said safety signal.
 4. A grinding machine according to claim 1, wherein said electrode means has a coolant nozzle of said coolant means for wetting said grinding wheel with a conductive coolant.
 5. A grinding machine according to claim 4, wherein said detecting means comprises: an amplifier for amplifying the difference of potential to generate an output voltage; a first setting means for setting a first reference voltage equal to the Output voltage appearing at an output terminal of said amplifier when a predetermined gap is formed between an outer peripheral edge of said grinding wheel and said workpiece when said feed means moves said wheel slide toward said workpiece at a rapid feed rate; and a first comparator circuit connected to said amplifier and said first setting means for comparing said output voltage with said first reference voltage to generate said control signal when the output of said amplifier voltage attains said first reference voltage, said control means causing said feed means to change the feed rate of said wheel slide from a rapid feed rate to a slow feed rate in accordance with said control signal.
 6. A grinding machine according to claim 5, wherein said detecting means further comprises: a second setting means for setting a second reference voltage equal to the output voltage appearing at an output terminal of said amplifier when said grinding wheel is in a desired wet state; and a second comparator circuit connected to the output terminal of said amplifier and said second setting device for comparing the output voltage of said amplifier with said second reference voltage to generate a confirmation signal, said control means being permitted in accordance with said confirmation signal to cause said feed means to move said wheel slide toward said workpiece.
 7. A grinding machine according to claim 5, wherein said electric power source is a direct current source.
 8. A grinding machine according to claim 5, wherein said electric power source is an oscillator and further comprises a rectifier connected to and between said amplifier and said first comparator.
 9. A grinding machine according to claim 8, wherein said electrode means comprises: a pair of electrodes mounted adjacent to a periphery of said grinding wheel, one of said electrodes being connected to said oscillator and the other of said electrodes being connected to said amplifier; an atomizer means for supplying an atomized coolant, and a coolant nozzle mounted adjacent to the pereiphery of said grinding wheel for wetting the same with said atomized coolant.
 10. A grinding wheel according to claim 8, wherein said detecting means further comprises: a second setting means for setting a second reference voltage equal to the output voltage appearing at an output terminal of said amplifier when said grinding wheel is in a desired wet state; and a second comparator circuit connected to the output terminal of said amplifier and said second setting device for comparing the output voltage of said amplifier with said second reference voltage to generate a confirmation signal, said control means being permitted in accordance with said confirmation signal to cause said feed means to move said wheel slide toward said workpiece.
 11. A grinding machine according to claim 9, wherein said electrode means comprises: an electrode mounted adjacent to a periphery of said grinding wheel, said electrode being connected to said amplifier; and a coolant nozzle mounted below said grinding wheel for supplying a conductive coolant, said coolant nozzle being connected to said oscillator.
 12. A grinding machine according to claim 11, wherein said detecting means further comprises: a second setting means for setting a second reference voltage equal to the output voltage appearing at an output terminal of said amplifier when said grinding wheel is in a desired wet state; and a second comparator circuit to the output terminal of said amplifier and said second setting device for comparing the output voltage of said amplifier with said second reference voltage to generate a confirmation signal, said control means being permitted in accordance with said confirmation signal to cause said feed means to move said wheel slide toward said workpiece.
 13. A grinding machine according to claim 1, wherein said coolant means is operable to supply coolant to said grinding wheeL in response to said control signal.
 14. A grinding machine according to claim 1, wherein said coolant means comprises: a first means for creating a desired coolant layer on said grinding wheel so as to connect said electrode means and said workpiece; and a second means for supplying a large amount of coolant to said grinding wheel and the workpiece in response to said control signal.
 15. A grinding machine according to claim 14, which further comprises a variable throttle valve for controlling an amount of coolant supplied to said first means. 